In order to obtain a high resolution electron microscope, it requires that an electron beam with a high current be irradiated onto a sample in such a condition that the beam is focused as small as possible. For that purpose, an electron gun to generate the electron beam is indispensably arranged with a higher luminance.
A Cold (Cathode) Field Emission (CFE) type electron gun is often adopted for a high luminance and high resolution electron microscope, the electron source of which gun is smaller in a light source size and higher in luminance than the other electron sources such as thermo electron emission and Schottky emission. In addition, with such electron gun, the energy spread of drawn electrons is very small. Upon the electron beam being focused with an electron optical system employing an electron lens, a blurring referred to as a chromatic aberration occurs in proportion to the energy spread of the beam. That is to say, the electron source having a smaller energy spread of emitted electrons permits the electron beam to be more finely focused, because it has a smaller chromatic aberration.
Conventionally, when such Cold (Cathode) Field Emission type electron gun is adopted for the electron gun, what employs a Butler electron lens composed of a drawing electrode 21 and an acceleration electrode 22 as shown in FIG. 2 is generally adopted. Hereupon, the Butler electron lens is a kind of electrostatic lens having the structure in which two pieces of circular electrode plates are disposed in parallel to each other on each side with regard to the electron source, in which the outer side of the two disks respectively is planner while the inner side opposed to such outer side is gradually thinned from the outer circumference thereof towards the center direction. An aperture, through which the electron beam passes, is provided between such two disks on each side. The electron lenses whose shapes are somewhat transformed from such two disks are also referred to as Butler electron lenses or Butler type electron lenses. Adopting the structure of such Butler electron lens advantageously allows the structure of the electron gun to be streamlined and the size of such gun to be relatively streamlined.
For the purposes that the aberration of the electron gun is abated and the luminance thereof is improved, such magnetic field superposition type electron gun is contrived as converging the electron beam employing a magnetic field lens in addition to an electrostatic lens, which electron gun is arranged in PTL 1 below by way of one example such that the electron source is disposed within the magnetic field of the magnetic field lens so that the electron beam emitted from the electron source is converged by use of the magnetic field lens whose focal length is short, thereby, allowing the electrostatic lens action to draw electrons from the electron source to be abated, with the result that the high current electron beam is realized with the aberration increase suppressed even when the captured angle of the beam′might be enlarged.